Reception difficulties occurring in an FM receiver, in particular a non-stationary receiver such as a car radio, are caused by a multitude of interference signals, for example co-channel or adjacent channel reception and noise, as well as multi-path and foreign interference, the latter caused, for example, by computers or industrial installations. As long as the amplitudes of the interference frequency or frequencies remain below a certain threshold, they are additionally suppressed by means of the usual technology (limiting amplifier and limiting demodulator). However, when they reach amplitudes comparable to the amplitudes of the useful signal, strong amplitude fluctuations, caused by signal heterodyning (known as beat) and simultaneously extreme phase modulation occur. This is in principle also applicable for PM (phase modulation) receivers. However, it will be described here by reference, by way of example, to an FM receiver.
Thus, there have been various attempts to detect the presence of an interference component in the total signal received by appropriate circuit technology or suitable methods and, since it is impossible to eliminate the interference signal because of the non-linear characteristics in the circuit, to correct the outgoing signal, at least during the time periods of maximum interference, and in this way to eliminate at least approximately the interference signal during these critical time periods as much as possible, at least to the extent that the auditory impression is noticeably improved.
An "interference suppression system" of this type, and its base in the state of the art, are described, for example, in German Published, Non-Examined Patent Application DE-OS 39 16 789. In this interference suppression system, an interference component is filtered out of an FM demodulating signal with the aid of an interference suppression device and, after further processing, a level control signal is generated with the aid of a control signal generator which causes interruption of the transmission of the FM demodulation signal or the FM detector signal to the subsequent stage. In this connection, the threshold or the preset level at which this control signal generator reacts can be automatically pre-determined by means of the received interference level which is contained in the FM demodulating signal. When the control signal generator is operated with the aid of a level maintenance circuit, a multiplex signal is therefore maintained at the actual value prior to the level control signal of the control signal generator during a certain length of time (correction phase), because of which a step-like characteristic is generated in the course of the multiplex signal due to the blanking of the interference. The known device recognizes the presence of interference with the FM demodulating signal with the aid of a high-pass filter, i.e. in the frequency spectrum which occurs. It is possible by means of this to exactly define the presence, but not the start and duration, of a phase interference event. Therefore, interference blanking of this type usually employs fixed blanking times which, as a rule, also because of characteristics of circuit technology, for example of the high-pass filter, last considerably longer than a large portion of the interference signals, such as are generated, for example, by ignition interference.
For this reason German Published, Non-Examined Patent Application DE-OS 39 16 789 does not address the length of the level control signal of the control signal generator, but only provides a "certain period of time" during which the level control signal is maintained.
The rigid fixing or the presetting of the blanking time (i.e. the correction phase) made necessary by the circuit technology necessarily involves a compromise with respect to the normally or preponderantly occurring interferences and their length. If the correction phase is too long, a portion of the usable signal is "given away", if it is too short, the interference blanking is partial, and a portion of the interference appears in the signal on the output side without having been affected. The detection of interference on the frequency side, i.e. before demodulation, by means of the high-pass filter necessarily depends largely on the frequency spectrum of the listener. Here, too, it requires adherence to compromises regarding the value or the frequency spectrum of the interference signals in question.
This leads in the end to a blanking of some interference events, while others are not blanked and still others result in unnecessarily prolonged correction phases. In turn, this can lead to interference repetition frequencies (unblanked and partially blanked interference signals), the frequency values of which are located in a lower frequency range than the original interference repetition frequency and therefore are very unpleasant for the listener. Thus the solution proposed in German Published, Non-Examined Patent Application DE-OS 39 16 789 is limited in its reaction to the multiplicity of very different interference signals to only the adaptation of the interference signal level. An "individual" addressing of the actually occurring interference signal in respect to the beginning and ending of the blanking or the correction phase is not possible. It thus leads only to a partially satisfying operation when this method or the circuit characteristics proposed there are used.
Differing from the method described above, it is also basically known in connection with the identification of interference signals within a received total signal to evaluate the amplitude range which occurs simultaneously with an interference phase modulation (interference phase characteristic is the first derivative d.phi./dt of the interference phase, which in the end appears as interference at the output of an FM demodulator). For example, German Published, Examined Patent Application DE-AS 21 42 172 discloses a blocking circuit in which the previously discussed problems of interference are resolved in that the low-frequency channel of the receiver is blocked by means of a "dead zone detector circuit" for the duration of amplitude dips if the incoming signal falls below a preset dip level, i.e. a preset threshold value. All that is disclosed therein regarding the value of this threshold is that it should take hold at a time when the signal field strength falls so far that "the usable signal sinks below the noise level". The size of the threshold value in this case can be set from the outside by affecting a related control signal, but then remains constant.
This known, species-defining method has the disadvantage that here, too, the selection of the threshold value of the dip level can only be based on an unsatisfactory compromise. Based on the nature of the occurring interferences, extremely rapid phase shifts and thus clearly audible distortions will be produced particularly by those interference signals which only lead to extremely amplitude dips in the total signal that are extremely small and while the amplitude of a received signal threshold value, remain above the set threshold value and consequently do not result in a correction, such as the blocking of the low frequency channel. They are in particular amplitude dips which are "small" in value but very steep and which can pass unharmed through the above mentioned "dead zone detector circuit" and therefore strongly impair the effectiveness of this blocking circuit.
In connection with a further method, disclosed in German Published, Non-Examined Patent Application DE-OS 34 46 529, the interfered received signal is also blocked out for the duration of the recognized interference. For this purpose the interference signal is generated from the received signal and is compared with a threshold value. Thus the beginning and end of an interference event are defined by the threshold value being exceeded in either direction. In this case an automatic adaptation of this threshold value to the frequency of interference in the received signal is provided in the sense that with increasing frequency of the interference, the threshold value is increased and in this way the circuit becomes increasingly less sensitive to the interference in order to assure at least adequate, even though qualitatively unsatisfactory, reception.
The last mentioned method in particular makes it clear that in connection with the problem of interferences and the distortions caused by them because of phase changes, it is at best possible to achieve a statistical improvement of the quality of the output signal by only introducing a constant or automatically adapting a threshold value for defining the correction phase.
In respect to individual interference, the correction phase defined in each of the known methods has been found to be insufficient, because its duration is only inadequately adapted to the duration of the individual interference event and therefore improvements of the quality of the output signal are only achievable with considerable restrictions.